In control animals fixed by perfusion, isolated fat emboli occur in scattered cerebral blood vessels. Their origin from connective tissue fat released in the course of autopsy is supported by (1) the histologic demonstration of fat on the surface of the brain, (2) the occurrence of fat emboli in pial arteries and in superficial cerebral blood vessels which have been torn, (3) the greater frequency of fat emboli when the brain is covered with oil, and (4) the prevention of such emboli when the brain is exposed under water. They are associated with post-mortem removal of the brain and do not reflect an intravital circulation of fat. In acute fat embolism induced by injection of oil, the emboli are separated by closely packed erythrocytes; under such conditions, a transport during life becomes evident.